Typically there is a phase angle difference between a sinusoidal current on a power grid as supplied by an alternating current generator and a voltage at the generator's terminals. This phase angle difference between the voltage and the current is due to the nature of the load on the power grid. For a purely resistive load (i.e., having no energy storage properties) the voltage and current are in phase, i.e., the current and voltage reverse their polarity simultaneously and a direction of power flow remains fixed and does not reverse.
For a purely reactive load the current and voltage are 90 degrees out of phase and the net power flow is zero as the power flows to and returns from the load due to the energy storage features of the reactive load. If the reactive load is purely inductive, the current lags the voltage by 90 degrees. A lag angle is between 0 and 90 degrees for a load that is both inductive and resistive. For a purely reactive capacitive load the current leads the voltage by 90 degrees. A lead angle is between 0 degrees and 90 degrees for a load with both resistive and capacitive properties. Thus the magnitude of the phase angle difference depends on the resistance, inductance and capacitance of the load to which the generator supplies power and on the characteristics and operating point of the generator.
A capacitive load tends to raise the system voltage (also referred to as the transmission line voltage) while an inductive load tends to lower the system voltage. Lightly loaded transmission lines also tend to raise the system voltage and heavily loaded lines tend to reduce the system voltage.
For a load with both reactive and resistive properties, the current phase angle (relative to the voltage phase angle, which for simplicity is assumed to be 0°) can be resolved into an in-phase component (i.e., in-phase with the voltage) and an out-of-phase component (i.e., a 90 degrees out-of-phase with the voltage). The component of the current that is in phase with the voltage delivers real or active power to the load. The component of current that is phase-shifted by 90 degrees from the voltage, referred to as reactive power, performs no useful work. The energy associated with this current flows from the generator to the load and then back to the generator, resulting in a net zero energy transfer.
The loading capability of a power transmission system is limited by thermal, dielectric and stability considerations. It is possible, but not necessarily practical, to control system real power directly. It is known that negotiating, injecting or drawing real power from a transmission or distribution bus affects the voltage at that bus. But this technique requires a real energy storage device, such as a battery or a superconducting storage device.
The transmission line power or system voltage can also be indirectly controlled by generating or absorbing reactive power from the transmission or distribution system. It is known that negotiating reactive power results in a more significant impact on system voltage than negotiating real power. DC capacitors can store and provide voltage/current to achieve reactive power control, but they do not store sufficient energy for controlling real power. Also, a system lacking adequate dynamic reactive power support (generating or absorbing reactive power) can result in voltage instability that may in turn lead to a voltage collapse, i.e., a blackout.
It is also known that as the reactive power increases it further reduces the amount of real power the system can carry. This limits the ability of the power system operator to meet the power demands of its users, as further increasing the amount of real power may cause the thermal limits of the transmission lines to be exceeded. The generation and control of reactive power in an electrical transmission system is therefore important to the overall power system efficiency and stability.
Generally, a capacitor supplies reactive power (i.e., supplies VARS) and an inductor absorbs reactive power (i.e., absorbs VARS). Thus if the transmission line voltage is low, capacitive reactive power needs to be supplied to the line to increase the line voltage. When the line voltage is high, inductive reactive power needs to be absorbed to lower the line voltage. Thus supplying or absorbing reactive power raises or lowers the line voltage, respectively.
Generating units can produce or absorb reactive power, although they cannot react quickly to power control demands. Capacitive and inductive compensators can be switched in and out of the transmission system, between a generator and a load, relatively quickly to generate or absorb reactive power as required to maintain the system voltage. Capacitors, capacitive loads and capacitive compensators are considered generators of reactive power. Inductors, inductive loads (e.g., transformers and motors) and inductive compensators are considered consumers or absorbers of reactive power.
If a transmission or distribution line segment supplying reactive power (using a capacitive reactive power compensator, for example) trips out of service, the amount of supplied reactive power declines and the system voltage drops on other segments of the transmission or distribution line that remain in-service. Conversely, a transmission line voltage can increase when loads are removed from the transmission line (automatic load rejection) or when the line is lightly loaded. Under the latter scenario, inductive reactive power compensators (consuming reactive power) are switched into service to reduce the transmission line voltage.
Electric power transmission systems are designed recognizing that the three power system parameters of impedance, voltage and power angle (i.e., a difference between a voltage phasor phase angle at a first end of a transmission line and a voltage phasor phase angle at a second end of the transmission line), cannot be controlled fast enough to accommodate dynamic system conditions. Furthermore, available control devices usually compensate or control only one of the three variables. Thus transmission and distribution systems having been designed with fixed or mechanically-switched series and shunt reactive compensations and voltage regulating and phase-shifting transformer tap changers to optimize line impedance, minimize voltage variation, and control power flow under steady-state or slowly changing load conditions.
The dynamic system problems are typically addressed by over-design, i.e., designing the system with generous stability margins to recover from worst case contingencies resulting from faults, line and generator outages, and equipment failures. This practice of over-design results in under utilization of the transmission system.
In recent years, energy demands, environmental considerations, right-of-way access, and cost issues have delayed the construction of both generation facilities and new transmission lines. This has necessitated a change in the traditional power system concepts and practices; better utilization of existing power systems has become imperative. But higher utilization of power transmission and distribution systems, without an appreciable degradation in the reliability of the supply of electric power is possible only if power flow can be controlled rapidly following dynamic system disturbances.
Power system stability is a measure of a power system's ability to provide electric power to meet load demand when power system conditions change. Various devices are in use to stabilize bulk-power transmission and distribution systems and to improve transient and dynamic stability of the power system. These devices, referred to generally as flexible AC transmission system (FACTS) devices can supply or absorb reactive power and in this way provide rapid voltage regulation and power flow control. FACTS devices include: static-var compensators (SVC), static synchronous compensators (STATCOMs), and thyristor-controlled series capacitors (TSCSs). Use of these devices to limit effects of the power system impedance changes permits loading of the transmission facilities to levels approaching their ultimate thermal capacity. These devices may supply or absorb reactive power to support the system voltage or provide power modulation to damp electromechanical oscillations. In any case, the FACTS devices control the voltage, impedance or phase angle (between the current and the voltage) on the power system.
STATCOM devices lack any substantial real energy storage and are simply voltage-sourced converters that absorb or supply reactive power to the grid via a step up transformer. In present devices, only transient energy storage is provided by a relatively small DC capacitor, common to all phases of the power system, that is used to exchange reactive power between the phases.
The present invention attempts to overcome problems of transmission and distribution line instability and control the transmission voltage by controlling reactive power on the transmission and distribution line.